1. Field of the Invention
The present invention relates to an image forming apparatus including a light emitting member provided with a plurality of light emitting elements which expose an image bearing member with light.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus, such as a digital copying machine and printer, generally employs a semiconductor laser as an exposure light source. A light beam emitted from the semiconductor laser is deflected by a light deflector including a rotating polygon mirror. The deflected light beam is imaged and scanned on a surface of a photosensitive drum via an fθ lens, and an electrostatic latent image is formed on the surface of the photosensitive drum.
On the other hand, many image forming apparatus available in recent years employ a light emitting diode array (hereinafter, referred to as “LED array”) as the exposure light source. The LED array includes a plurality of light emitting diodes (hereinafter, referred to as “LEDs”) arrayed in line. The LED array is held by an LED head. A plurality of light beams emitted from the LED array are each imaged on the surface of the photosensitive drum via a rod lens array, and an electrostatic latent image is formed on the surface of the photosensitive drum.
In general, a conjugation length of the LED array is about 10 mm. The LED array is an optical system which is easily affected by deviation of a focus position from the surface of the photosensitive drum in a direction of an optical axis, as compared to a laser optical system having a relatively large focal length. Thus, it is necessary to accurately maintain a constant distance between the photosensitive drum serving as an image bearing member and the LED head holding the LED array.
Japanese Patent Application Laid-Open No. 2005-335074 discloses an image forming apparatus including an automatic adjusting device for automatically adjusting the distance between the photosensitive drum and the LED head with use of a motor, and a measurement device for measuring a deviation amount of an imaging position through measurement of density of a measurement pattern recorded on a recording medium. Based on a measurement result from the measurement device, the automatic adjusting device displaces adjusting members provided at both end portions of the LED head, to thereby adjust the distance between the LED head and the photosensitive drum. According to the configuration disclosed in Japanese Patent Application Laid-Open No. 2005-335074, the LED head can be positioned with high accuracy and without an adverse effect of changes with the passage of time.
Further, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-335074includes a pressing device for pressing a longitudinal center portion of the LED head to adjust the distance between the LED head and the image bearing member. The LED head initially has a projecting shape, in which the longitudinal center portion thereof is spaced away from the image bearing member as compared to the end portions thereof. The pressing device adjusts a pressing force to be applied to the center portion of the LED head, to thereby adjust the distance between the LED head and the image bearing member. Accordingly, even an LED head having a large width can be positioned with high accuracy over the entire width of the LED print head so that the LED head can be mounted to the image forming apparatus.
However, it is extremely difficult to adjust the distance between the LED head and the image bearing member for positioning over the entire range of an exposure width with high accuracy, and ensure the positioned state over a long period of time. In the configuration disclosed in Japanese Patent Application Laid-Open No. 2005-335074, even when the LED head can initially be positioned with high accuracy through the adjustment at the time of assembly, the positioned state cannot be maintained with high accuracy in some cases due to adverse effects of vibrations that may occur after the positioning, physical distribution, and long-term use.